In addition to what was said above you might cosider installing this; AutoDisconnect I have found it has helped increase the battery life on my N900.
I discovered 3G to be a real battery killer, when at home connected to the WLAN and idle I was getting far more time than when I was connected to 3G at work under the same conditions- this app when configured correctly through the easy to use menu will downgrade an idle connection to save power.

I have aslo found this application to be useful in figuring out whats draining my battery, batterygraph.

Welcome the N900 club - I hope that after a small period of adjustment you will be amazed at the versitility of this tiny device on a daily basis, as I still am.